1. Field of the Invention
This invention is directed to wellbore tractors and, in one particular aspect, to a tractor system useful in a non-vertical wellbore to continuously move a tubular string, a wireline, a cable, or coiled tubing.
2. Description of Related Art
In vertical wellbores and semi-vertical wellbores which are not highly deviated, wirelines, cable, coiled tubing, tubular strings and tools introduced into the wellbore move down into the wellbore by the force of gravity. Gravity effects such movement when the wellbore is nearly vertical or is not deviated from vertical to such an extent that gravity forces the items against the wall of the wellbore rather than further into it.
Cable or wireline reaches a deviation threshold (e.g. for certain systems a deviation of about 70.degree. from the vertical, e.g. wireline systems) at which gravity no longer provides the necessary force and resulting tension to move the cable or wireline down and through a wellbore.
To a certain extent tubulars and coiled tubing can be pushed through a deviated wellbore, even part of a horizontally or upwardly directed wellbore; but there is a limit to the length of coiled tubing that can be pushed in this manner. When compressive loads in a tubular become large enough, the tubular helically locks up in the wellbore (cased or uncased) and further movement is prevented. This is known as "helical lockup."
A variety of prior art wellbore tractors have been developed. For example U.S. Pat. No. 4,463,814 discloses an anchor-and-ram-unit assembly for propelling a drilling tool in a wellbore. The assembly has two anchor assemblies each with anchor feet that are hydraulically activated to move out and engage an interior wellbore wall. When a first anchor assembly is anchored, a movable piston moves by hydraulic fluid pressure down to move the drilling tool. When that piston reaches the limit of its movement, the second anchor assembly is set with its anchor feet engaging the wellbore. The first anchor assembly is then disengaged and a movable piston of the second anchor assembly is hydraulically activated to move the drilling tool. This system operates at relatively low speed and does not provide continuous motion of the drilling tool.
U.S. Pat. No. 4,095,655 discloses a low speed system for moving a drill bit laterally in a drilling operation. The system employs elastomers and reinforcing material operated in tension and has four hydraulically operated force cells on a thrust mandrel. Two of the force cells are lateral force cells expandable in a lateral direction, but of substantially fixed dimensions in the radial direction. The other two force cells are radial force cells, expandable radially, but having substantially constant lateral dimensions. In a typical operating sequence, with the lead lateral cell expanded, the lead radial cell is expanded to engage the walls of the borehole securely and effectively anchor itself to the borehole at that point. The lead lateral cell is then deflated, and the rear lateral cell correspondingly expanded to move the rear radial cell forward a distance corresponding to the difference in length between the rear lateral cell in its contracted and expanded positions. The rear radial cell is then expanded to engage the borehole walls, while the lead radial cell is contracted. Then the lead lateral cell is expanded while the rear lateral cell is contracted, to thereby move the lead portion of the mechanism forward a distance corresponding to the difference in length between a lateral cell in the expanded and contracted positions.
U.S. Pat. No. 4,223,737 discloses a method for pre-assembling a series of tubulars above the point of insertion into a wellbore in a horizontal mode and then moving the string of tubulars with a prime mover into a wellbore. The prime mover does not enter the wellbore.
Various known prior art wellbore tractor systems do not provide continuous movement. They operate in a stop-start mode or with a significant dwell time between anchoring and movement steps. Consequently the speed of such systems is relatively low, e.g. twenty feet or less per hour.
There has long been a need, recognized by the inventors of the present invention, for an efficient, effective and fast wellbore tractor system. There has long been a need for such a system which provides continuous or nearly continuous pulling of a cable, wireline, tubular string or coiled tubing. There has long been a need for such a system which, in certain embodiments, is small enough to pass through a small inner diameter tubular, but then is operable to work effectively in a larger inner diameter tubular. There has long been a need for such a system which operates effectively in a cased or uncased wellbore.